My 403 came with an attachment box 161279 and is about 5 5/8" x 7 1/2" x 2.18". It is probably similar to
https://www.worthpoint.com/worthoped...set-1691363178 or numerous others. My parts list says attachment set for the 401 is 161173 and according to
http://www.singersewinginfo.co.uk/simanco/161173/ was a plastic box, but does not give dimensions.
Janey - Neat people never make the exciting discoveries I do.